74 ELEMENTARY PHYSIOLOGY. 



As we have already learned, a tooth 

 e f 



a Tooth. 



f is not a bone. It does not belong to 



the skeleton. The teeth are instru- 

 ments or organs of the digestive system. The 

 structure of a tooth is an interesting study. To help 

 us understand it, we are provided, in the Anatomical 

 Aid, with the means of completely dissecting that 

 is, separating into its parts, the structure of a tooth. 

 In the manikin of a tooth, the little projecting 

 ridges at the top (1) are called tubercles. The por- 

 tion above the gum (2) is called the crown. This 

 is covered by a thin layer of enamel, the hardest 

 material in the body. The tooth is mainly composed 

 of a substance called dentine, or ivory (9). At (6) 

 and (7) the roots of the tooth are shown. At (12) 

 blood-vessels and nerves are seen to enter into 

 the tooth. When an opening occurs in the body of 

 the tooth, from decay, this nerve is exposed to the 

 air and the action of food particles, and toothache 

 is the result. 



The third step in digestion is insaliva- 



Work of the 



salivary lion. Really, mastication and insahva- 

 < lamis. . Qn are performed at the same time. 



While the food is being chewed by the teeth, a liquid 

 is mixed with it in the mouth. This liquid is spe- 

 cially prepared for this purpose, from the blood, by a 

 number of organs called salivary glands. By the 

 way, let us not forget that all the substances of the 

 body are prepared from the blood. A gland is an 

 organ which secretes which means separates some 



